In gas phase polymerization processes, a gaseous stream containing one or more monomers is passed through a fluidized bed under reactive conditions in the presence of a catalyst. A polymer product is withdrawn from the reactor, while fresh monomer is introduced to the reactor to react with the catalyst and replace the removed polymer product. A gas phase fluidized bed reactor can include a reaction zone and a so-called velocity reduction zone. The reaction zone can include a bed of growing polymer particles, formed polymer particles, and a minor amount of catalyst particles fluidized by the continuous flow of gaseous monomer and diluent to remove heat of polymerization through the reaction zone. A portion of the gases within the reactor can be re-circulated via a cycle gas stream. This cycle gas stream can be passed through a heat exchanger, where at least a portion of the heat of polymerization can be removed, and then compressed in a compressor and returned to the reaction zone.
While the manufacture of a particular polymer product may be fairly well understood, the transition from the manufacture of one polymer product to another in an efficient manner may still be problematic. For example, process conditions that favor the efficient production of one polymer product may not be conducive to the formation of a different product. Different polymer products often require not only the use of different conditions, but also the use of different catalysts. Moreover, disparities in the characteristics of the polymer products themselves can add to the difficulties in efficient transitioning. Thus, the development of processes and procedures for efficiently transitioning from the manufacture of one polymer product to another continues to be an area of research.
One method of improving the efficiency of such transitions includes the addition of a polymerization neutralizing agent (sometimes referred to as a “kill agent’) to the polymerization. Typical kill procedures require the reactor to be opened, purged of hydrocarbons, emptied of polymer and catalyst particles, cleaned, and reloaded with the removed bed or a new bed to provide a “seedbed” of polymer. This process is time consuming, expensive, and allows impurities, such as moisture and air, to enter the reactor. Such impurities necessitate another time consuming procedure to remove, which typically involves a nitrogen purge to reduce the impurity levels to less than 10 ppm, before restarting the reactor.
It has been recognized that polymerization reactors may be vulnerable to sheeting and/or fouling during the critical initial stage(s) of a polymerization reaction (before the reaction has stabilized) even if each such initial stage is performed in the presence of a continuity additive (CA). Typically, the concentration of CA in a reactor is too low to eliminate this vulnerability if the CA is introduced during the initial stage(s) of the polymerization reaction. Thus in another method, a continuity additive is pre-loaded into the polymerization reactor or a mixture of a CA and a seed bed are pre-loaded into the reactor. This is sometimes referred to as “quick seedbed replacement.”
Nevertheless, efficient methods for transitioning between the production of desired polymer products remain a need, particularly where different catalysts are used to prepare the desired products.